I've had one boy who went outside who never stopped coming in to use his litter tray; and one boy (The Lodger) who actually started using his tray before he went out, and coming in during the day, although he had been using outside previously.
And who could blame them? Inside, a nice warm, safe, dry tray always cleaned carefully by the Litter Fairy (Huh!) and outside, cold, wet, muddy, hard crusted earth and other cats or people watching. It's probably a no-brainer to some cats.
People swear by gradually staging the litter tray form its current position , to near the door, to outside the door (if there's a covered space) over a few weeks, say. (Perhaps also sprinkling some litter on a flowerbed to scent-guide them.) Also, it might help to dig over eg flowerbeds so that the earth is loose. The current weather isn't helping and most earth right now is packed hard and quite difficult to shift even with a fork.
I don't mind it, really. It saves having to deal with neighbours re poo - or pick up poo from outside when you see it - and always gives you a good idea of the cat's health. But that's just me.
You may see a difference with the arrival, some day, of spring and summer and outside drying up.